In a round of calls Sunday, Kerry, Japanese Foreign Minister Fumio Kishida and South Korea's Kim Sung-hwan all agreed the North must understand "that it will face significant consequences from the international community if it continues its provocative behavior," according to a summary of the calls from the U.S. State Department.

Earlier Sunday, North Korea announced that its leader, Kim Jong Un, "has made an important decision" that would strengthen the country. The brief statement on the state-run news agency KCNA provided no details, but it said the decision was made at a meeting of the reclusive Stalinist state's Party Central Military Committee.

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Across the Demilitarized Zone, South Korean President Lee Myung-bak called on his government to be prepared for a possible test. Lee paid a visit to the underground bunker that serves as the South's crisis management center, his press office reported.

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North Korea has conducted two previous nuclear tests, in 2006 and 2009, and proclaimed itself a "nuclear state" in 2012. U.S. officials told CNN last week that the North appeared to be ready test another nuclear device "at any time."

U.S. analysts believe the 2006 test had a yield of about 1 kiloton -- comparable to the explosive power of about 1,000 tons of TNT -- while the second was roughly 2 kilotons, National Intelligence Director James Clapper told a Senate committee in 2012.

By comparison, the bomb the United States dropped on Hiroshima in 1945 was roughly 15 kilotons.